This invention relates to flame-retardant polyester textile filaments and fibers, and is more particularly concerned with melt-blending ethylene terephthalate polymer with a flame-retardant agent and extruding the melt to form filaments.
Phenylphosphonic dichloride can be reacted with resorcinol to produce a polyphosphonate (herein designated PRP) which is useful as a flame-retardant additive in plastics. However, when used as an additive in polyethylene terephthalate, PRP has been found to cause photoyellowing in polyester fabrics which are exposed to ultraviolet light. Under similar conditions, fabrics made of commercial polyethylene terephthalate filaments (no PRP additive) have excellent whiteness. A polyphosphonate prepared by reacting phenylphosphonic dichloride with 4,4'-dihydroxydiphenyl sulfone, instead of resorcinol, causes less discoloration but is too expensive.
Poly(phosphonate-phosphate) copolymers can be prepared by reacting phenylphosphonic dichloride and phenyldichlorophosphate with resorcinol to produce poly[(m-phenylene phenylphosphonate).sub.q (m-phenylene phenylphosphate).sub.r ], wherein q and r are integers. The phosphate component makes the flame-retardant additive less expensive but does not overcome the discoloration problem, and cross-linking reactions are difficult to avoid. The presence of cross-linked polymer in the flame-retardant polyester composition can increase the viscosity to such an extent that satisfactory melt-spinning is impracticable.
Diphenyl methylphosphonate can be reacted with resorcinol to produce a poly(m-phenylene methylphosphonate) having phenyl methylphosphonate end groups (herein designated PRM/P). When it is used as a flame-retarding agent in polyethylene terephthalate, heating at 283.degree. C. for 30 minutes under nitrogen causes a drastic reduction in the polymer relative viscosity to less than half the original relative viscosity. Compositions of this type have not been considered suitable for use in producing commercial textile filaments.